1. Field of the Invention
This invention relates to telephone station set circuits and, in particular, to circuits responsive to digital control signals for implementing key telephone functions at the station set.
2. Description of the Prior Art
Since the advent of key telephone systems several years ago, there has been an increasing demand that more and more service features be made available to the customer. Implementation of these additional features, such as call hold, operator recall, and the like, has generally required additional circuits within the station set itself as well as additional lines between the station set and a centrally located controller. At some point both size and cost considerations have rendered this method of implementation prohibitive. For example, it was not uncommon to have as many as 25 pairs of lines connecting the station set to the controller. With the frequent shift of personnel utilizing key telephone apparatus, the time and expense involved to effect a relocation has become excessive.
To circumvent these problems, the recent trend has been toward implementation of key telephone service features by means of signal multiplexing techniques. Multiplexing numerous signals for transmission over a given conductor pair has permitted a substantial reduction in the number of pairs which must be utilized for interconnection. Although this approach has alleviated to some extent the cost and space problems, it has been less than totally successful. As before, the addition of other customer service features has required modifications and changes in the circuits.
With the addition of other features, the amount of power required to drive these circuits has greatly increased. In many applications it is desirable to have the telephone system totally divorced from the commercial power system. For such applications many of the revised system designs have been found unsuitable. Other system designs, specifically directed toward operation on telephone system supplied power, have not proved successful because of excessive power drain caused by the numerous added features.
Accordingly, it is one object of the present invention to reduce the number of signal leads into and out of the station set which, in turn, facilitates concentration of the number of leads into a cross connection network.
Another object is to implement special customer service features by data stream control.
A further object of the present invention is to utilize large scale integrated circuit technology to reduce the cost and size of the circuits, to decrease the amount of power required to drive the circuits, to improve overall circuit reliability, and to reduce the number of lumped element components required.
Still another object is to simplify key design and thereby make it more reliable by incorporating a single contact, nonlocking button in conjunction with a pair of light emitting diode lamps, one indicating the button-in-use status and the other indicating line status.
Yet a further object of the present invention is to use a direct drive for the light emitting diodes thereby reducing the number of components required and the amount of power supply noise.